In the production of oil from subterranean oil reservoirs by various flooding techniques, especially waterflooding, it has become a common expedient to add various polymeric thickening agents to the water in order to increase its viscosity to a point where it approaches that of the crude oil which is to be displaced so as to improve the displacement of the oil from the reservoir. The use of polymers for this purpose is often stated to be for mobility control.
Another problem which arises in the various flooding processes is that different strata or zones in the reservoir often possess different permeabilities so that displacing fluids enter the high permeability or "thief" zones in preference to zones of lower permeability where significant quantities of oil may be left unless measures are taken to plug the high permeability zones wholly or partly and so divert the displacing fluid into the low permeability zones. Mechanical isolation of the thief zones has been tried but vertical communication among reservoir strata often renders such measures ineffective. Physical plugging of the high permeability zones by cements and solid slurries has also been attempted with varying degrees of success but here, one serious drawback is the possibility of permanently closing still productive horizons.
From these early experiences, the desirability of designing a viscous slug capable of sealing off the most permeable layers so that the floodwater would be diverted to the underswept, tighter regions of the reservoir, became evident. This has led to the use of oil/water emulsions, gels and polymers for controlling the permeability of the formations in a process frequently referred to as "profile control", a reference to the control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed have included oil/water emulsions, gels, e.g. lignosulfonate gels and polymers, with polymers being the most extensively applied in recent years.
Profile control agents are also used to control the permeability of the high permeability regions in conjunction with miscible displacement or thermal processes of enhanced oil recovery (EOR), such as nitrogen and carbon dioxide flooding or steam flooding.
Among the polymers so far examined for controlling the permeability of the high permeability regions are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates, as described, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680. These organic polymers may be used in either their uncrosslinked forms or as crosslinked metal complexes, e.g. as described in the patents previously mentioned.
One problem which has persisted with the use of the various organic polymers as permeability control agents is that their stability may not be wholly satisfactory at all the elevated temperatures commonly encountered in oil-bearing formations. There is therefore a continuing need for materials which will block the high permeability regions of oil-bearing formations over an extended range of temperatures, particularly in reservoirs which are at a high temperature.
Proposals have been made for the use of inorganic polymers, especially silicates, as permeability control agents and proposals have been made for the use of inorganic silicates in this way. For example, U.S. Pat. Nos. 4,009,755 and 4,069,869 disclose the use of inorganic silicates for this purpose. In the permeability control method described in these two patents, an organic polymeric permeability control agent such as a cross-linked polyacrylamide or polysaccharide is first injected into the reservoir, followed by an aqueous solution of an alkaline metal silicate and a material that reacts with the silicate to form a silicate gel which plugs the high permeability regions in the formation. An alkaline metal silicate is typically used as the source of silica and the gelling agent is usually an acid or acid forming compound such as a water soluble ammonium salt, a lower aldehyde, an aluminum salt or an alkaline metal aluminate.
The problem, however, with many inorganic silicates is that their solutions are often quite viscous and stable only under alkaline conditions. As soon as conditions approach non-alkalinity, silicate gel is formed. Although this is the desired reaction for plugging the formation, it may occur prematurely, before the solution has had an adequate opportunity to enter the high permeability regions of the formation, cutting off the possibilities for further injection of plugging material. This is obviously undesirable and it would represent a distinct improvement if the silicates could be made more stable under a wider range of pH conditions. The use of silica sols has been considered but these often contain particles with diameters in excess of 10 microns, too large to penetrate many reservoir rock pores.